1. Field of the Invention
This invention relates to optical devices that respond to input light at one wavelength by emitting light at a different wavelength, and more particularly to an upconversion waveguide and fabrication method that can be monolithically integrated on a semiconductor substrate.
2. Description of the Related Art
The excitation of rare earth ions in crystals and glasses has been used in the past to make infrared lasers in which the pump or excitation wavelengths are shorter than the laser output wavelength. For several ion types an energy sharing process takes place between excited ion pairs or triplets that result in a single ion being excited to an energy level greater than that of the pumping photons. This makes possible the operation of "upconversion" lasers that have outputs at wavelengths shorter than that of the pump source, including numerous different wavelengths in the ultraviolet and visible regions when driven by infrared sources. (Although typically used to produce an output with a wavelength shorter than the pump, the ion energy sharing can also be used with an appropriate nonradiative decay to produce a longer wavelength output; such "downconversion" systems are included within the generic term "upconversion" as used herein.)
Erbium doped fluoride crystals have recently been used as high power visible upconversion lasers. Such a device is discussed, for example, in U.S. Pat. No. 5,008,890 to McFarlane, assigned to Hughes Aircraft Company, the assignee of the present invention. Unfortunately, these devices must be operated at cryogenic temperatures, which requires a fairly complex and expensive system. In the McFarlane patent mentioned above, for example, the operating temperature range is restricted to 15.degree.-120.degree. K. The need for low temperatures arises from the fact that the excited ion population decays from the desired upper laser level via nonradiative mechanisms that are associated with the crystal lattice vibrations. This loss is reduced at lower operating temperatures because of a reduction of the phonon density in the host crystal.
An alternate to cryogenic operation is to offset the above loss by increasing the pump power density, and thereby compete directly with the losses by a more rapid pumping of the upper laser energy level. This has been achieved with single mode glass optical fibers that confine the energy of the pump source to a cylindrical region on the order of 5 microns in diameter. Because of the small fiber cross-section, the intensity of the pump beam within the fiber can be very high and thus avoid the need for cryogenic operation. An example of an upconversion device of this type is presented in Whitley et al., "Upconversion Pumped Green Lasing in Erbium Doped Fluorozirconate Fibre", Electronics Letters, Vol. 27, No. 20, 26 September 1991, pages 1785-1786. Visible laser operation has been reported at room temperature using erbium, thulium, holmium and praseodymium as dopant ions in special single mode fibers fabricated from heavy metal fluoride glass. While theoretically it might also be possible to obtain room temperature upconversion with a crystal, presently available cutting and polishing techniques are not capable of producing crystals with faces than less than about 200 square microns, which is not small enough for the high pump beam intensities necessary for room temperature operation.
Although the use of optical fibers avoids the need for cryogenic operation, it still has significant limitations. It is difficult to reliably draw the glass fiber into the small core dimensions that are required, and the fibers tend to degrade over time.